The yeast that grows on fermenting fruit and that is the main food source of the fruitfly Drosophila melanogaster can sometimes harbour harmful microbes. How do fruitflies recognize which yeasts are safe and which are coated in dangerous micro-organisms? Geosmin is an aversive odourant that is produced by numerous harmful microbes, and a new study finds that the circuitry activated by this compound is functionally segregated and involves a single class of olfactory sensory neuron. This circuit is conserved across the Drosophila genus and provides a mechanism for avoiding potentially dangerous environments.